This invention generally relates to polymer thick film conductive compositions and coatings. In particular, the invention is directed to such compositions, which are suitable for making variable resistive elements such as those used in position sensors.
Electrically resistive or conductive polymer thick film compositions have numerous applications. Polymer thick film (PTF) resistive compositions are screenable pastes, which are used to form resistive elements in electronic applications. Such compositions contain conductive filler material dispersed in polymeric resins, which remain an integral part of the final composition after processing.
Resistive compositions are used as resistive elements in variable resistors, potentiometers, and position sensor applications. A resistive element is, in most cases, printed over a conductive element, which acts as a collector element. In position sensing applications, a metallic wiper slides over the resistive element. The wiper can slide back and forth for several million cycles over the collector and resistive elements during the lifetime of the electronic component. For accurate position sensing, the wiper should give continuous electrical output throughout the life of the sensor.
The durability of these position sensing elements depends on the mechanical properties of both the resistor and the conductive film. The polymer thick films tend to wear out after several million cycles of sliding with a metallic contactor over the element at extreme temperature conditions typically seen in an environment such as an automotive engine compartment. Therefore, polymer resistive and conductive compositions having excellent mechanical properties and wear resistance are required for performance and signal output in these applications.
In addition to good mechanical properties, these materials should also have good thermal properties. Polymer thick films show a decrease in storage modulus as temperature is increased. A sharp decrease in mechanical properties is observed near the glass transition temperature. In addition to loss in modulus, these materials also tend to show an increase in coefficient of thermal expansion, which increases significantly above the glass transition temperature (Tg). When used in, for example, motor vehicles, a position sensor is exposed to high temperatures in under the hood applications.
At these temperatures, resistive elements show a high rate of wear due to a decrease in modulus properties. In addition to the surrounding temperature, a still higher temperature is observed at the interface between the metallic wiper and the resistive element surface due to frictional heating. In some cases, these temperatures can approach the glass transition temperature (Tg) of the resistive material and can cause loss of the material's mechanical properties, which adversely affect signal output.
A prior art resistor composition is as follows:
Prior Art Composition
ComponentWeight (%)Polyamide imide21.58Carbon black5.47N-methyl pyrrolidone72.95
One way to improve mechanical properties of a resistive film is to incorporate fillers, such as short fibers, in these films. The presence of fibers of relatively large dimension creates an electrically heterogeneous surface. This results in non-linear electrical output in contact sensor applications. Even when the size of the fibers is in the order of a few microns, the surface is still electrically and mechanically heterogeneous. A dither motion at high frequency on a surface region where these fibers are absent can create large wear.
Another problem with using fibers with greater than 10 volume percentage is they can significantly wear the metallic contactor. This wear is accelerated if these fibers are protruding from the surface. Therefore, there is a need in the art for a conductive wear resistant coating or resistor composition and cured film with enhanced mechanical and thermal properties that exhibits homogeneous surface electrical characteristics and good signal quality.